This invention generally relates to projection television systems and more specifically is directed to providing a distortion-free image on the screen of a projection television.
A projection television system is generally comprised of a cathode ray tube (CRT), or a plurality of CRT's as in the case of a color projection television receiver for providing the three primary colors, and a large reflecting screen for presenting an enlarged image of the CRT-generated video image. Each CRT includes an electron gun, a target coated with a phosphor upon a surface directly facing the impinging electron beam, various mirror and/or lens combinations to achieve the desired optical projection, and a transparent face plate through which the image is projected upon the large projection screen external to the CRT's envelope. The projection screen is typically located in front of a viewer in a plane generally perpendicular to the viewing direction of the viewer.
The spatial relationship between the various projection television system components and the position of the viewer with respect thereto require careful consideration in system design. For example, positioning the CRT's of the television receiver and the projector in front of the projection screen will result in partial obstruction of the screen from a position directly in front thereof, at which position it is most desirable for the viewer to be positioned. In addition, the viewer may not be located between the CRT/projector combination and the projecting screen for then the projected light path will be interrupted and the display of video information will be rendered impossible. To overcome these restrictions, two basic approaches are generally taken in projection television systems. One involves lowering the projector position relative to the viewing screen while the other involves raising the projector so as to position it in a relatively high location. In either case, the goal is to remove the projector from the general viewing area in providing an unobstructed field of view over the entire sector forward of the projecting screen. By thus locating the projector, projection of the reproduced picture onto the enlarged screen is carried out obliquely from a position either below or above the axis of the projection screen.
By thus locating the image projector, optical distortions arise in the video image as a result of vertical off-axis pointing. In order to eliminate such distortions, one approach taken in the prior art has used a vertical correction deflecting coil in addition to the normal vertical deflecting coil. A parabolic signal having a horizontal frequency is applied to this correction deflecting coil to remove a bow distortion from the projected video image. Vertical correction deflecting coils of this type are expensive and add substantially to the complexity of the projection television system.
Another approach to video image vertical linearity correction in an off-axis projection television system is disclosed in U.S. Pat. No. 4,198,591 to Ohmori. This vertical deflecting circuit employs a horizontally derived correction signal which is either added to or multiplied with the verticl sawtooth wave deflection signal for regulating vertical sweep rate. This circuit is intended to correct for bow distortion in the CRT horizontal scan lines as projected on the viewing screen which appears as horizontal image distortion across the width of the viewing screen.
The present invention provides an improved means for correcting for linear distortion in the vertical direction of a video image displayed on an off-axis projection television screen.